The behavior of electromagnetic radiation is altered when it interacts with charged particles. Whether these charged particles are free, as in plasmas, nearly free, as in conducting media, or restricted, as in insulating or semi conducting media—the interaction between an electromagnetic field and charged particles will result in a change in one or more of the properties of the electromagnetic radiation. Because of this interaction, media and devices can be produced that generate, detect, amplify, transmit, reflect, steer, or otherwise control electromagnetic radiation for specific purposes.
The behavior of electromagnetic radiation interacting with a material can be predicted by knowledge of the material's electromagnetic materials parameters ∈ and μ where ∈ is the electric permittivity of the medium, and μ is the magnetic permeability of the medium. These parameters represent a macroscopic response averaged over the medium, the actual local response being more complicated to describe and generally not necessary to describe the electromagnetic behavior.
Reflection and transmission at the interface between two media are governed by the index of refraction η and impedance z of each medium. The index η and the impedance z are directly related to the reflection and transmission properties of a slab of material, and hence are the observable quantities that correspond directly to the electromagnetic performance of materials. The index of refraction η and the impedance z can be expressed in relative terms in relation to corresponding properties for free space as:η=[(∈μ)/(∈0μ0)]1/2 z=(μ/∈)1/2/(μ0/∈0)1/2 where the subscript 0 indicates free space values associated with a vacuum. Air has very nearly the index of refraction and impedance of vacuum. Thus, the relative index of refraction and the relative electromagnetic impedance z of air are often taken to be equal to unity. Note that the permittivity and permeability can be found from the index and the impedance using the above relations, as ∈=η/z and μ=ηz.
In addition to having low material losses, a material that is electromagnetically “transparent” will have both its index of refraction and impedance numerically close to that of the surrounding medium. Such a material is valuable for many applications. For example, airplanes may have a collision detection radar system mounted near their “nose.” This system operates inside a composite dome known as a radome that has a shape optimized for aerodynamic properties. The radar system must compensate for the lensing effects of the shaped radome composite material, which typically has a relative index of refraction that is significantly greater than unity. Such compensation requires effort and expense, and is subject to error.
By way of additional example, structural materials may be used to embed a sensor such as an array of antennas in a wireless communications device. Reflection and refraction effects in these structural materials are likewise undesirable. In both of these applications, material requirements, irrespective of their electromagnetic reflection and refraction properties, include physical properties such as strength, ductility, and resistance to heat, cold, and moisture. The prior art has had limited success in satisfying these needs.
Materials and methods for generally minimizing electromagnetic reflection and maximizing transparency have been proposed. For example, materials have been proposed that have a high absorption of incident radiation at microwave and other frequencies. In addition to preventing transmission of radiation, the strong absorbance of these materials often leads to a substantial reflected component. As a result, use of these materials is usually accompanied by irregular material shapes and surface angles required to direct the reflected component in a desired direction. The required irregular surface angles and shapes significantly limit the utility of such materials and methods.
Also, the prior art has employed particular naturally occurring media that may be found in nature or that can be formed by known chemical synthesis and that may have a low level of electromagnetic reflection over a particular frequency range. Use of such media is disadvantageously limited to these particular frequency ranges. Also, it is difficult to find media with significant permeability at RF and higher frequencies. These media may also be structurally unsuitable for many applications.
Previous study of the effects of so-called “artificial dielectric” materials on electromagnetic waves has been performed. For example, artificial dielectric materials based on arrays of substructures that collectively have a desired response to electromagnetic radiation have been studied. These arrays, which need not necessarily be periodic in nature, have in common that the dimensions and spacing of the scattering elements are less than the wavelengths over which the composite material will operate. It is found that by averaging the local electromagnetic fields over such a structure, an effective permittivity (and/or permeability) function can be applied that roughly describes the scattering properties of the composite. The procedure that arrives at this description is known in the literature as “effective medium theory.”
An example of a prior art artificial dielectric material is the “rodded” medium, used as an analogue medium to study propagation of electromagnetic waves through the ionosphere [See, e.g., R. N. Bracewell, “Analogues of an Ionized Medium”, Wireless Engineer, 31:320-6, December 1954, herein incorporated by reference]. An artificial medium based on conducting wires or posts has a dielectric function identical to that describing a dilute, collisionless neutral plasma. Accordingly, as used herein a medium based on conducting wires will be referred to as a “plasmonic” medium. More recently, artificial plasmonic media have been proposed using, for example, a periodic arrangement of very thin conducting wires. See, e.g., J. B. Pendry et al., “Extremely low frequency plasmons in metallic mesostructures”, Physical Review Letters, 76(25):4771-6, 1996; see also D. R. Smith et al., “Loop-wire for investigating plasmons at microwave frequencies,” Applied Physics Letters, 75(10):1425-7, 1999; both of which are incorporated herein by reference.
Other recent examples of artificial dielectrics include the use of random arrangements of metal “needles” suspended in a foam structure as a “lens” with an index of refraction greater than unity. Many foam-like materials have a refractive index approximately equal to unity. Adding needles serves to increase the index for low-frequency RF radiation as with radio astronomy. These materials, however, are not acceptable for applications requiring a degree of mechanical strength.
To date, these prior art efforts have not been successful in providing materials that have a low reflectance and good transparency at a desired wavelength in addition to having advantageous structural mechanical properties.
Unresolved needs in the art therefore exist.